The present invention relates to an exhaust gas purification system that purifies particulate matters from the exhaust gas discharged by diesel and other internal combustion engines using a continuous regeneration type diesel particulate filter device and also to a control method thereof
In the same way as for NOx, CO, and also HC etc., restrictions on the volume of particulate matters (hereinafter referred to as “PM”) discharged from diesel internal combustion engines grow severe every year. Techniques for collecting this PM in a filter known as a diesel particulate filter (hereinafter referred to as “DPF”) and for reducing the quantity thereof by discharging externally have been developed.
DPFs for collecting this PM include a monolithic honeycomb form wall flow type filter made of ceramic, a fiber form type filter made of fiber shape ceramic or metal, and so on. An exhaust gas purification system using these DPFs are installed on the way of the exhaust passage of an internal combustion engine, similarly to the other exhaust gas purification systems, for cleaning exhaust gas generated in the internal combustion engine before discharging the same.
These DPF devices include a continuous regeneration type DPF device wherein an oxidation catalyst is installed upstream of the DPF, a continuous regeneration type DPF device wherein the PM combustion temperature is lowered by the effect of a catalyst supported on a filter with catalyst and PM is burned by the exhaust gas, etc.
The continuous regeneration type DPF device wherein the oxidation catalyst is installed upstream of the DPF uses the fact that the oxidation of PM by NO2 (nitrogen dioxide) is executed at a lower temperature than the temperature at which PM is oxidized with oxygen in the exhaust gas. This continuous regeneration type DPF device is composed of an oxidation catalyst and a filter. NO (nitrogen monoxide) in the exhaust gas is oxidized to NO2, by an oxidation catalyst supporting platinum or the like on the upstream side. PM collected by the filter on the downstream side is oxidized by this NO2 to CO2 (carbon dioxide). Thereby, PM is removed.
Besides, the continuous regeneration type DPF device of filter with catalyst is composed of a filter with catalyst such as cerium oxide (CeO2). In this DPF device, PM is oxidized by a reaction (4CeO2+C→2Ce2O3+CO2, 2Ce2O3+O2→4CeO2, etc.) using O2 (oxygen) in the exhaust gas by means of the filter with catalyst, within the low temperature range (on the order of 300° C. to 600° C.). PM is oxidized by O2 (oxygen) in the exhaust gas, within the high temperature range (equal or higher than the order of 600° C.) which is higher than the temperature where PM is burned with O2 in the exhaust gas.
In this continuous regeneration type DPF device of filter with catalyst and the like, the oxidation catalyst is also installed on the upstream side and oxidation and removal of PM is stimulated by raising exhaust gas temperature through oxidation reaction of unburned HC and CO in the exhaust gas. At the same time, the emission of unburned HC and CO into the atmosphere is prevented.
However, these continuous regeneration type DPF devices also cause the problem of exhaust pressure rise by the clogging of this filter. When the exhaust gas temperature is equal or higher than 350° C., PM collected by this filter (DPF) is burned continuously and cleaned, and the filter regenerates itself. However, in the case of low exhaust gas temperature and in an operating condition of an internal combustion engine where the emission of NO is low, for example, in the case where the low exhaust gas temperature state such as idling of internal combustion engine, low load/low speed operation continues, the oxidation reaction is not stimulated as the exhaust gas temperature is low, the catalyst temperature lowers and the catalyst is not activated and, moreover, NO lacks. Consequently, the aforementioned reaction does not occur and the filter can not be regenerated through oxidation of PM. As a result, PM continues to be accumulated in the filter and the filter clogging progresses.
As a measure against this filter clogging, it has been conceived to forcibly burn and remove the collected PM by forcibly raising the exhaust gas temperature, when the amount of clogging has exceeded a predetermined amount. As for means for detecting such filter clogging, there are some methods such as a method for detecting by the differential pressure before and after the filter, and a method for detecting through determination of the PM accumulation quantity by calculating in accordance of a map data in which the collecting quantity of PM to be collected is previously set based on the engine operation state. Besides, as means for exhaust gas temperature raising, there is a method by injection control of the fuel injection into a cylinder, or a method by fuel control in the direct fuel injection in the exhaust pipe.
In the case of this injection control of the fuel injection into a cylinder, when an exhaust gas temperature is lower than the active temperature of an oxidation catalyst supported by an oxidation catalyst or a filter set on the upstream side of the filter like the case of low speed or low load, an exhaust gas temperature necessary for PM combustion is not obtained. Therefore, the temperature of exhaust gas is raised through performing multi injection (multistage injection). Moreover, when the temperature of the exhaust gas becomes higher than the active temperature, post injection (posterior injection) is performed separately from normal fuel injection to burn the fuel in the exhaust gas by an oxidation catalyst and raise the temperature of the exhaust gas to a temperature at which the PM collected in a filter burns or higher. Thereby, the collected PM is burned and removed to regenerate the filter.
However, because the fuel for the post injection is not burned in the cylinder of an engine, injected fuel mixes in the oil of the engine through the joint of a piston ring or the like to cause oil dilution due to the fuel. The oil dilution causes deterioration of the viscosity of the engine oil etc. Therefore, when fuel dilution of a certain level or more occurs, it influences the durability of the engine and a problem occurs in the reliability of the engine. Therefore, it is necessary to keep the fuel dilution quantity within a constant level until replacement of the engine oil. However, because the way of being used of a vehicle is variously changed, the distance at which the collecting quantity of PM reaches a limit to require forced regeneration is changed. Therefore, there is a problem that the fuel dilution quantity cannot be controlled.
Moreover, when the problem of the oil dilution is left as it is, abrasion or seizure of the sliding portion of an engine is caused. Therefore, it is important to solve the problem. Furthermore, to keep an oil replacement distance, it is important to perform the control for minimizing unburned fuel injected through the post injection in forced regeneration.
For example, when a vehicle travels for a considerable travel distance, the fuel mixed in oil is evaporated. Therefore, oil dilution is improved. To solve the problem that oil is excessively diluted, the following exhaust gas purification device for internal combustion engine is disclosed in Japanese patent application Kokai publication No. 2003-120390. This device judges the period from a subordinate injection date and time of unburned fuel of previous time up to the subordinate injection date and time of this time. When this period is longer than a predetermined period required for all unburned fuel diluted in lubricant to evaporate, a large retardation value of subordinate injection is taken to increase the unburned fuel to be added to a catalyst. When the former period is shorter than the latter period, dilution is not performed.
However, when the forced regeneration processing is carried out while a vehicle travels, the control of the post injection at the time of transition at a change of operations of an engine is difficult compared to a stable condition while the vehicle stops. That is, even if loads are changed and an engine temperature rises in a transition state, it is difficult to avoid unnecessary injection (unnecessary strike) that post injection is performed. As a result, the frequency of oil dilution due to unburned fuel increases.
That is, an operation state is not stabilized by automatic forced regeneration during vehicle traveling. Therefore, the fuel injection quantity in the post injection increases. Moreover, because oil is diluted due to a large amount of unburned fuel post-injected for DPF regeneration, the oil viscosity is deteriorated. Furthermore, the oil replacement distance decreases. Therefore, it is not preferable that the forced regeneration processing is frequently performed during vehicle traveling.
However, in the case of forced regeneration control while a vehicle stops, the operation state is stabilized because of the stop, the injection quantity of fuel decreases, and oil dilution is comparatively small. Therefore, it is considered to stop a vehicle and then perform the forced regeneration without performing the forced regeneration control while the vehicle travels. That is, when the operation condition such as idling while the vehicle stops is stabilized, the forced regeneration is performed by performing post injection of an injection quantity smaller than the load while the vehicle travels to raise temperature. Thereby, oil dilution is decreased compared to the case of regeneration control while a vehicle travels. Thereby, problems of the oil dilution are solved.
As one of the solutions, a method is considered in which a filter is regenerated by using a lamp etc. and thereby notifying a driver that forced regeneration is necessary when a filter is clogged at a predetermined value and the driver receiving the notification stops a vehicle and then operates a manual regeneration switch at the driver seat to perform the forced regeneration control.
However, when using the above method, in the case of a vehicle having many patterns for traveling in a low-speed large-load operation state, manual regeneration is frequently necessary. Therefore, a lamp for prompting manual regeneration is turned on, that is, the interval for requesting a driver to press a manual regeneration switch becomes short and the frequency for the request becomes frequent. Therefore, a problem of making the driver feel botheration occurs.